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Flow visualization is the art of making flow patterns visible. Most fluids (air, water, etc.) are transparent, thus their flow patterns are invisible to the naked eye without methods to make them this visible. Historically, such methods included experimental methods.
Particle image velocimetry (PIV) is a non-intrusive optical flow measurement technique used to study fluid flow patterns and velocities. PIV has found widespread applications in various fields of science and engineering, including aerodynamics, combustion, oceanography, and biofluids.
Tufts, mini-tufts, or flow cones can be applied to a model and remain attached during testing. Tufts can be used to gauge air flow patterns and flow separation. Tufts are sometimes made of fluorescent material and are illuminated under black light to aid in visualization. Tufts are also used on aircraft in flight.
Schlieren photography is a process for photographing fluid flow. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.
Their motion can be observed and recorded to locate air flow features such as boundary layer separation and reattachment. Tufting is, therefore, a technique for flow visualization. They are used during flight testing to study air flow direction, strength, and boundary layer properties. [citation needed]
Schlieren flow visualization of a Lockheed SR-71 Pratt & Whitney J58 engine inlet at Mach 2. Schlieren flow visualization is based on the deflection of light by a refractive index gradient [4] The index gradient is directly related to flow density gradient. The deflected light is compared to undeflected light at a viewing screen.
Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the schlieren and schlieren photography methods that perform a similar function. Shadowgraph is a type of flow visualisation.
The Mach–Zehnder interferometer's relatively large and freely accessible working space, and its flexibility in locating the fringes has made it the interferometer of choice for visualizing flow in wind tunnels [8] [9] and for flow visualization studies in general.